When joining steel materials used in various industrial fields, as is well known, use is generally made of the means of bolting them in addition to various joining means using welding. In such bolting means, higher tensile strength of the steel materials is being promoted due to the pursuit of greater economy and technical advances. Much use is being made of friction joining means using high strength bolts offering high reliability in joints and superior in work efficiency as well particularly in the fields of civil engineering and construction.
As the high strength bolts used for friction joining, for example, extensive use is being made of the sets of friction joint use high strength hexagonal bolts, hexagonal nuts, and flat washers defined by the JIS-B-1186 of the Japan Industrial Standard and the sets of structural use Torque-Shear type high strength bolts, hexagonal nuts, and flat washers of JSSII-09 of the Japan Society of Steel Construction. Under these circumstances, recently, in particular along with the larger scale of civil engineering and construction structures, development of high strength bolts having bolt tensile strengths of 1200 N/mm2 or more is being strongly sought.
A conventional high strength bolt is for example produced by quenching and tempering a low alloy steel such as SCM435 defined by JIS-G4105. When using such a machine-use tough steel for actual use, however, with a bolt having a tensile strength of 1200 N/mm2 or more, the phenomenon of “delayed fracture” occurs where the bolt suddenly breaks after the elapse of a certain time from fastening even with use under the yield stress, so such bolts cannot be used as the all important joining parts for buildings and bridges. Therefore, the increase in strength of bolts has currently halted at 1100 N/mm2 class refined steel.
Further, in the past, for a steel material for use for a high strength bolt, for example as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-191762, Japanese Unexamined Patent Publication (Kokai) No. 3-173745, etc., there is a method of production taking note of the fact that the fracture face of a bolt due to delayed fracture exhibits grain boundary fractures and comprising reducing the P, S, and other impurities in the chemical composition of the steel material so as to strengthen the grain boundaries and, from the viewpoint of controlling the structure, adding Mo and Cr to enable high temperature quenching of 400° C. or more to impart properties not easily leading to fracture even with invasion of hydrogen, the cause of delayed fracture, into the steel material. In particular, reducing the impurity P, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-9653, is an extremely effective technique for reducing the P segregating at the grain boundaries and improving the grain boundary strength.
Even in the above method of production, however, if more than a certain concentration of hydrogen invades into the steel material, delayed fracture will occur, so further advances are desired for further improving the delayed fracture resistance property of a bolt.
Therefore, to further improve the delayed fracture resistance property of a bolt, it is effective to make it difficult for hydrogen to invade into the steel material or reduce the concentration of hydrogen at the old austenite grain boundaries. For example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-70890, a technique has been proposed for suppression of the invasion and diffusion of hydrogen into the steel material by the simultaneous addition of Si and Ni to the steel material. Not only does such addition of Si impair the cold forging property of a bolt, however, but also the addition of Ni raises the cost.
Further, Japanese Unexamined Patent Publication (Kokai) No. 7-278735 discloses bolt steel having a tensile strength of 1200 N/mm2 or more responding to the above request and superior in delayed fracture property. Further, the above publication describes as specific measures (1) adding together elements Mo, Cr, and V for causing remarkable secondary hardening at the time of tempering so as to enable the achievement of a strength of 1200 N/mm2 or more even with high temperature tempering at 450° C. or more, (2) adding more than 0.35% to 1.0% of V to make the old austenite grain size No. 10 or less and tempering the steel at a high temperature of 450° C. or more to cause the precipitation of V carbides and nitrides forming hydrogen trap sites, (3) reducing the impurities P, S, and Si segregating at the grain boundaries to strengthen the old austenite grain boundaries, in particular reducing the amount of P to 0.008% or less, so that the bolt will not easily corrode even in a harsh corrosive environment of dipping in 36% hydrochloric acid, the amount of hydrogen invading the steel is remarkably reduced, and simultaneously the delayed fracture resistant property is improved, and (4) reducing the element Si strengthening the solid solution of ferrite to secure an amount of softening at the time of spheroidizing annealing and enable cold forging without reducing the amount of addition of other alloy elements improving the delayed fracture resistant property.
In this case as well, however, even when tempering at a temperature of 450° C. or more, when refining the steel to a tensile strength of 1400 N/mm2 or more, there is the problem that the rate of occurrence of delayed fracture becomes high. Further, depending on the shape of the high strength bolt, in particular the thread shape, there is also the problem of a high possibility of early occurrence of delayed fracture.
Further, Japanese Examined Patent Publication (Kokoku) No. 6-89768 (set of high strength bolt, nut, and washer) discloses a high strength bolt designed so that the seat surface of the head portion is a conical surface tilted 90° to 150° with respect to the bolt axial center. In this case, due to variations unavoidable in manufacture, error unavoidable in installation, etc., the seat surface of the head portion cannot be given a uniform contact pressure and stress concentrates locally. In such a case, there is the problem that the bolt yield strength and delayed fracture resistant property fall. Further, a large amount of trouble and time are required compared with the past for forming the seat surface of the head portion and the washer receiving the seat surface of the head portion. Further, there are two types of seat surfaces and production control becomes complicated, so the cost is increased. Further, management of the two types of washers becomes necessary at the time of installation and there is the problem of a fall in the bolting efficiency.
A high strength bolt, however, as shown in FIG. 4, is comprised of a head portion 1 and a shaft portion 2 formed integrally by a steel material, but the location suffering from delayed fracture is mainly the thread part 3 cut into the shaft portion 2. This thread part 3 experiences a large concentration of stress. Further, it experiences a larger plastic deformation with respect to fastening in the bolt axial center X—X direction with a high axial force. Therefore, it is known from numerous research, delayed fracture occurs starting from such a location. Further, the thread part 3 of a conventional high strength bolt, for example, an M22 bolt defined in Japan Industrial Standard JIS B 1186 (F10T JIS metric coarse thread bolt), as shown in FIG. 5, normally has facing flanks 4a and 4b of threads 4 cut into it at an equidistant pitch L (2.5 mm) having an angle θ of for example 60°, has each thread 4 of the shaft portion 2 having a trapezoidal shape obtained by cutting a pointed peak 5 at H/8 from the tip, where H is the height from a bottom 5a of the pointed peak 5 (H=2.165 mm), and has each valley bottom 4c of the threads 4 formed into an arc-shaped curve by setting points of transition Q1 and Q2 between facing flanks 4a and 4b and the valley bottom 4c to H/3 from the bottoms 5a of the pointed peaks 5 and drawing an inscribed circle 6 contacting the points of transition Q1 and Q2 of the flanks 4a and 4b and having a radius R1 of H/6, where the center point O of the inscribed circle 6 is set at a height of H/12 from the positions of the points of transition Q1 and Q2. Due to this, the concentration of stress acting on the thread part 3 is reduced. Even with this, however, the coefficient of stress concentration acting on a center part M of a valley bottom 4c of the threads 4 is 2.54. When pulling uniformly in the bolt axial center X—X direction by a standard bolt tension corresponding to a tensile strength of 1200 N/mm2 or more, large plastic strain still occurs at the valley bottom of the threads and the occurrence of delayed fracture of the high strength bolt at a tensile strength of 1200 N/mm2 or more is still not dealt with.
Therefore, the present inventor engaged in various research considering the above situation and as a result discovered the relation between the bolt tensile strength and tempering temperature and the relation between the bolt tensile strength and carbon equivalent calculated from the chemical composition of the steel material and set the chemical composition of the steel material and performed quenching and tempering so as to satisfy these two relations and thereby perfected a high strength bolt superior in delayed fracture resistant property able to be improved in bolt tensile strength to 1200 N/mm2 or more and a steel material for the same.